System and method of electric motor fault detection

ABSTRACT

A system for detecting faults in a motor includes a drive circuit, a detection circuit, and a controller. The drive circuit is configured to apply a drive signal to a motor. The detection circuit is configured to detect a response signal generated when the drive signal is applied to the motor. The controller is configured to determine a motor fault based on a comparison of the response signal to an expected signal for the drive signal applied to the motor. The drive signal is selected to generate a rotating magnetic field in the motor with a rotation-frequency greater than a maximum mechanical-response-frequency of the motor.

TECHNICAL FIELD OF INVENTION

This disclosure generally relates to a system for detecting electricalfaults in a motor, and more particularly relates to applying a drivesignal selected to generate a rotating magnetic field in the motor witha rotation-frequency greater than a maximummechanical-response-frequency of the motor.

BACKGROUND OF INVENTION

It is known to equip a vehicle with an electric drive actuator or motorfor providing torque to the engine/driveline/wheels, and optionallygenerating electricity when mechanically driven. Various failure modesof the motor include internal shorts or opens of phases in the motor.Before the motor is used by the vehicle, it is advantageous to test themotor without actually operating the motor. In addition, it isadvantageous to track key motor characteristics over time to detectdegradation before a significant failure occurs. Techniques to detectopens and shorts have been proposed that operate the motor (e.g. attemptto generate torque) and then detect faults such as over currents,controller faults associated with lack of control of the motor, orplausibility checks on two currents being equal and opposite or all zeroindicating an open phase. These faults can have a variety of potentialcauses that are not related to faults in the motor. For instances, twocurrents being equal and opposite is a natural condition that occursevery sixty degrees of rotation and may exist continuously during stall.Controller faults can be related to current sensors or gate signals notfunctioning correctly. Furthermore, these checks can lack fidelity andallow a degrading machine to not be correctly identified until asignificant fault occurs, which can result in a stranded operator of thevehicle.

SUMMARY OF THE INVENTION

In accordance with one embodiment, a system for detecting faults in amotor is provided. The system includes a drive circuit, a detectioncircuit, and a controller. The drive circuit is configured to apply adrive signal to a motor. The detection circuit is configured to detect aresponse signal generated when the drive signal is applied to the motor.The controller is configured to determine a motor fault based on acomparison of the response signal to an expected signal for the drivesignal applied to the motor. The drive signal is selected to generate arotating magnetic field in the motor with a rotation-frequency greaterthan a maximum mechanical-response-frequency of the motor.

In another embodiment, a method of detecting faults in a motor isprovided. The method includes the step of applying a drive signal to amotor. The method also includes the step of detecting a response signalthat arises when the drive signal is applied to the motor. The methodalso includes the step of determining a motor fault based on acomparison of the response signal to an expected signal for the drivesignal applied to the motor. The drive signal is selected to generate arotating magnetic field in the motor with a rotation-frequency greaterthan a maximum mechanical-response-frequency of the motor.

Further features and advantages will appear more clearly on a reading ofthe following detailed description of the preferred embodiment, which isgiven by way of non-limiting example only and with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described, by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a diagram of system for detecting faults in motors inaccordance with one embodiment;

FIG. 2 is a graph of a signal present in the system of FIG. 1 inaccordance with one embodiment;

FIG. 3 is a graph of a signal present in the system of FIG. 1 inaccordance with one embodiment;

FIG. 4 is a graph of a signal present in the system of FIG. 1 inaccordance with one embodiment; and

FIG. 5 is a flowchart of a method of operating the system of FIG. 1 inaccordance with one embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a non-limiting example of a system 10 for detectingfaults in an electric motor, hereafter the motor 12. The system 10described herein was conceived while developing ways to test electricmotors used in vehicles such as automobiles. However, it is recognizedthat the system 10 described herein could be used in non-automotiveapplications such as industrial applications. The motor 12 is depictedas similar to a three-phase brushless motor, presumably with a permanentmagnet rotor. However, the teachings presented herein are applicable toother types of motors such as inductance, synchronous reluctance, andwound field synchronous motors, and with a number of phases other thanthree, as will be recognized by those in the art.

The system 10 includes a drive circuit 14 configured to apply a drivesignal 16 to the motor 12. Typically, the drive circuit 14 would be afull-wave type drive able to apply positive and negative differentialvoltages to the phases 26 (PA, PB, and PC) of the motor 12.Alternatively, the drive circuit 14 may be a controlled-current typedrive that is configured to inject a desired current into each of thephases 26 instead of applying a voltage onto each of the phases 26.

The system 10 includes a detection circuit 20 configured to detect aresponse signal 18 that arises when the drive signal 16 is applied tothe motor 12. By way of example and not limitation, the response signal18 in this example are the currents IA, IB, and IC flowing in the phasesPA, PB, and PC, respectively. Alternatively, if the drive circuit 14were a controlled-current type drive, then the detection circuit 20 maypreferably be configured to determine the voltages commanded to orapplied to each phase, e.g. VA, VB, and VC. If the detection circuit 20is configured to determine current, e.g. IA, IB, and IC, the detectioncircuit 20 may include, for example, a small value current senseresistor for each phase, or Hall-effect type current sensors, as will berecognized by those in the art.

The system 10 includes a controller 22 configured to determine a motorfault based on a comparison of the response signal 18 to an expectedsignal 24 for the drive signal 16 applied to the motor 12. The responsesignal 18 is reported to the controller 22 by the detection circuit 20as illustrated, and the drive signal 16 is output by the drive circuit14 in response to control signals from the controller 22. The controller22 may include a processor (not shown) such as a microprocessor or othercontrol circuitry such as analog and/or digital control circuitryincluding an application specific integrated circuit (ASIC) forprocessing data as should be evident to those in the art. The controller22 may include memory, including non-volatile memory, such aselectrically erasable programmable read-only memory (EEPROM) for storingone or more routines, thresholds, captured data, and the expected signal24. The one or more routines may be executed by the processor to performsteps for determining if signals received by the controller 22 indicatethat there is a fault (i.e. functional problem) with the motor 12 asdescribed herein. The motor fault may be indicated to an operator of avehicle via, for example, a message displayed on a driver informationcenter, or by illuminating an icon on an instrument panel of thevehicle.

The system 10 may include a rotor angular position sensor, hereafter theangle sensor 30, configured to provide an angle signal 28 to thecontroller 22 indicative of the angle of the rotor (not shown) relativeto the windings (not shown) or stator (not shown) of the motor 12, aswill be recognized by those in the art. The angle signal 28 may be usedto determine or compensate the expected signal 24, which can be recalledfrom memory by the controller 22, or may be determined (i.e. estimatedor calculated) by the controller 22 as a function of the angle signal28. That is, the response signal 18 that is generated in response to thedrive signal 16 being applied to the motor 12 is influenced by theangular position of the rotor relative to the windings or stator of themotor 12. As such, the angle signal 28 is useful to adjust what theexpected signal 24 is supposed to be, and thereby provide a betterreference for comparison to the response signal 18 by the controller 22.Alternatively, the response signal can be calculated using the angularposition in order to create a response signal that is fixed fordifferent rotor positions and thus be compared to a constant expectedresult. Furthermore, the detection circuit may provide signals to thecontroller 22 that can be further modified before a comparison is madeto the expected result 24.

The system 10 described herein overcomes the short comings of the priorattempts to perform motor tests by performing a diagnostic test prior tousing the motor 12 to generate any substantive torque. The diagnostictest described herein may be performed during, for example, initialpower-up of the system 10. In general, the diagnostic testadvantageously applies normal operating voltages (e.g. 30V PWM'd from a325V supply) to the motor 12 instead of the relatively lowvoltage/current test signals (e.g. 5V) suggested in the prior art.Applying normal operating voltages is advantageous as high voltages andcurrents may better reveal dielectric insulation breakdown in thewindings of the motor 12, and/or better reveal a localized highresistance caused by fracture fatigue in wire of the windings orterminations of the motor 12. However, in order to avoid movement of themotor, the diagnostic test is performed in a relatively short timeinterval, and using a drive signal that does not cause the motor torotate.

When operating the motor 12 to generate typical torque levels, therotational speed that the magnetic field within the motor 12 is rotatedis equal to the rotor's rotation speed, and at a specific phase relativeto the rotor's field-dictated by the desired torque, desired efficiency,and motor characteristics. If a rotor is not rotating (i.e. not moving),and a rotating magnetic field with a sufficiently high rotational speedis applied to the motor 12, the average torque will be substantiallyzero, with little to no rotor movement as the mechanical inertia willeffectively filter out the alternating component of the torque. As such,a test may be performed if the drive signal 16 is selected to generate arotating magnetic field in the motor 12 with a rotation-frequency 202(FIG. 2) greater than a maximum mechanical-response-frequency of themotor 12. As used herein, the term ‘rotation-frequency’ refers to thefrequency at which the magnetic field produced by the winding of themotor is rotated by applying AC voltages of the same frequency to eachphase of the motor, which can differ from the rotor's rotational speedbecause of the design of the motor. Furthermore, the maximummechanical-response-frequency is a value of the rotation-frequency 202(FIG. 2) where any incremental movement of the motor 12 does notinfluence the load to which the motor 12 is coupled and the responsesignal 18 is also not influenced. That is, the motor 12 is effectivelystalled when the rotation-frequency 202 is greater than a maximummechanical-response-frequency of the motor 12 even though the amplitudeof the drive signal 16 is similar to the amplitude used to operate themotor 12 when the rotation-frequency 202 is lower. The maximummechanical-response-frequency may be determined by empirical testingand/or computer modeling, and may take into consideration the expectedmechanical load on the motor 12.

FIG. 2 illustrates a non-limiting example of a graph 200 of the drivesignal 16 that is suitable for testing the motor 12. In this example,the rotation-frequency 202 of the drive signal 16 illustrated in FIG. 2is about eight-hundred Hertz (800 Hz). The sinusoidal currents aredeveloped by providing a ramped sinusoidal voltage to the motor 12 so asnot to needlessly excite transient currents which could negativelyimpact the phase current measurements by requiring prohibitively longtime periods to reach steady state. The drive signal 16 is typicallygenerated by a pulse-width-modulation (PWM) pattern at, for example,ten-thousand Hertz (10 kHz), but only the underlying 800 Hz fundamentalsignal content is shown. That is, the drive signal 16 shown in FIG. 2does not show the PWM signal used to generate the drive signal 16, whichwould have a much higher frequency than what is illustrated. Assuggested before, a current regulator could still be used to drive thecurrents, though care must be taken to ensure proper protection if onephase is partially shorted or opened.

The graph 200 of the drive signal 16 includes a ramp-up portion 210after the drive signal 16 starts at time 0 seconds, a ramp-down 220portion before the drive signal 16 ends at time 0.015 seconds, and atest portion 230 between the ramp-up portion 210 and the ramp-downportion 220. The test portion 230 is between time 0.005 seconds and0.010 seconds. The ramp rates of drive signal 16 during the ramp-upportion and the ramp-down portion are configured or selected to reduceor avoid current transients that can cause radiated or conductedemissions in excess of desired emission levels, which may be set byvehicle manufactures and/or government agencies and would unnecessarilycause the test portion time to be increased. If the ramp rates are tooslow, then the total test time for the diagnostic test performed byapplying the drive signal 16 may be unacceptably long, causing anunacceptable delay before the motor 12 is available for operation togenerate torque.

As suggested above, one embodiment of the system 10 described herein hasthe detection circuit 40 configured to measure current (e.g. IA, IB, IC)flowing in one or more phases 26 of the motor 12, and the responsesignal 18 is based on current flowing in one or more phases of the motor12 during the test portion 230 of the drive signal 16. The controller 22may sample data from the detection circuit 20 and process that sampleddata so that the response signal 18 can be characterized by a singlevalue such as an average magnitude 304 of current flowing in a phase(e.g. PA, PB, BC) of the motor 12.

FIG. 3 illustrates a graph 300 of a non-limiting example of the responsesignal 18 generated in response to the drive signal 16 of FIG. 2 beingapplied to a non-limiting example of the motor 12. Because the angularposition of the rotor influences the effective inductances of eachphase, there are different average magnitudes of the currents IA, IB,and IC. Accordingly, the angle signal 28 can be used by the controller22 in order to determine an expected signal 24 for the response signal18.

The average magnitude 304 may be determined by the controller 22sampling data from the detection circuit 20, and calculating analgebraic average of the absolute value of any or all of the currentsIA, IB, IC, or calculating a root-mean-square (RMS) value. Severaloptions exist for detecting opens and shorts. The most basic check,which requires the least amount of processing power and also provides anidentification of the specific phase in question, is to analyze thecurrents IA, IB, IC individually. For instance, absolute maximum/minimumor mean of the absolute value can provide sufficient information todetermine if the current is of appropriate size. Currents that are toolow indicate that an open-circuit or high impedance condition exists inthe motor in a particular phase (PA, PB, PC). If multiple phases areopen-circuit, then no current would flow, which can also be detected.Currents that are too high indicate that a short-circuit or lowimpedance condition exists. For extreme cases of terminal shorts, thesystem 10 may include overcurrent protection (not shown), as will berecognized by those in the art.

By way of further example and not limitation, the controller 22 maycalculate an average current 304 in one of the phases 26 to be eightAmperes (8A). The controller 22 may be configured to indicate a motorfault if the average magnitude 304 is less than an open-circuitthreshold 306, for example, two Amperes (2A). Similarly, the controller22 may be configured to indicate a motor fault if the average magnitudeis greater than a short-circuit threshold 308, for example, thirtyAmperes (30A). The values selected for the open-circuit threshold 306and the short-circuit threshold 308 may be done by empirical testing, orby engineering analysis.

FIG. 4 illustrates a graph 400 of a non-limiting example of a q-axiscurrent IQ and a d-axis current ID derived from a transformation of thecurrents IA, IB, and IC by Eq. 1 in order for the characteristiccurrents 404 (e.g. IQ, ID) of the motor 12 to be viewed in a rotorreference frame 402.

$\begin{matrix}{{\begin{pmatrix}{IQ} \\{ID} \\{IO}\end{pmatrix} = {\frac{2}{3}\begin{pmatrix}{\cos (\theta)} & {\cos \left( {\theta - 120} \right)} & {\cos \left( {\theta + 120} \right)} \\{\sin (\theta)} & {\sin \left( {\theta - 120} \right)} & {\sin \left( {\theta + 120} \right)} \\\frac{1}{2} & \frac{1}{2} & \frac{1}{2}\end{pmatrix}\begin{pmatrix}{IA} \\{IB} \\{IC}\end{pmatrix}}},} & {{Eq}.\mspace{11mu} 1}\end{matrix}$

where θ (i.e.—theta) is indicated by the angle signal 28.

In this non-limiting example, the response signal 18 is characterized asa characteristic current 404 derived from currents IA, IB, IC flowing ineach phase PA, PB, PC of the motor 12 and the angle signal 28. Thecharacteristic current may be indicated by a value such as an averagemagnitude of the q-axis current IQ, eight Amperes (8A) for example.Alternatively, the d-axis current may also be used. The expected value24 may be determined by the controller 24 recalling from memorypre-programmed values. The controller 22 may use the angle signal 28 toadjust the value recalled from memory to compensate for varying degreesof inductive coupling caused by changes in the angular position of therotor within the motor 12. The controller 22 may be further configuredto indicate a motor fault if the characteristic current 404 differs fromthe expected value 24 by more than a difference threshold, indicatedhere as the threshold 404. It is appreciated that there may also beanother threshold below the characteristic current 404 in order toprovide a ‘window’ or range of values of the characteristic current 404does not indicate a motor fault.

The controller may also be configured to determine a present inductancevalue (e.g. LA, LB, LC; FIG. 1) of a phase (PA, PB, PC) of the motor 12based on the response signal 18 during the test portion 230 of the drivesignal 18. The present inductance value LA may be calculated based upona phase difference between the drive signal A (i.e. VA), the current IA,and the rotation frequency 202, as will recognized by those in the art,or the relationship of voltage to current amplitude through an inductor.The amplitude of the current can be easily found based on a minimumvalue and a maximum value of the rotor reference frame's DQ currentflowing in the motor during the test portion 230 of the drive signal 16.The expected value 24 for any calculated present inductance value may bea baseline inductance value that is a preprogrammed value programmedinto the controller 22 at the time the controller 22 was manufactured,or a learned value based on inductance values calculated and stored whenthe controller 22 and other electronics were initially connected to themotor 12. Alternatively, an inductance value of the motor 12 may bebased on inductance values indicated in alternative reference framessuch as the rotor reference frame 402 or the stator reference frame (notshown).

As such, the controller 22 is configured to compare the presentinductance value LA, LB, LC to a baseline inductance value, and indicatea motor fault if the present inductance differs from the baselineinductance value by more than an inductance change threshold. Forexample, if the present inductance LA is determined to be ten-point-fivemilli-Henrys (10.5 mH), and the expected value 24 is 10.0 mH based on abaseline value of 10.0 mH for the rotor angle indicated by the anglesignal 28, and the inductance change threshold is 0.3 mH, then since thepresent inductance LA differs from the expected value by more than theinductance change threshold, a motor fault should be indicated becausethe inductance value has changed too much.

FIG. 5 illustrates a non-limiting example of a method 500 of detectingfaults in a motor 12. The method 500 may be initiated (START) when thecontroller 22 first receives power, and if all of the tests are passed(END) then the method 500 indicates that no motor faults have beendetected.

Step 510, APPLY DRIVE SIGNAL, may include applying a drive signal 16 tothe motor 12. As suggested above, the drive signal 16 is selected togenerate a rotating magnetic field in the motor 12 with arotation-frequency 202 greater than a maximummechanical-response-frequency of the motor 12.

Step 520, DETECT RESPONSE SIGNAL, may include detecting a responsesignal 18 that arises when the drive signal 16 is applied to the motor12.

Steps 530-580 describe various ways of determining a motor fault basedon a comparison of the response signal 18 to an expected signal 24 forthe drive signal 16 applied to the motor 12. Preferably, the responsesignal defines or used to calculate an average magnitude 304 of currentflowing in a phase of the motor 12. It should be recognized thatopen-circuit, short-circuit, and other fault conditions can bedetermined for an individual phase of the motor 12 or various groups ofthe phases 26.

Step 530, AVERAGE MAGNITUDE<OPEN-CIRCUIT THRESHOLD, may includeindicating a motor fault by jumping to step 590 if the average magnitudeis less than the open-circuit threshold.

Step 540, AVERAGE MAGNITUDE>SHORT-CIRCUIT THRESHOLD, may indicating amotor fault by jumping to step 590 if the average magnitude is greaterthan the short-circuit threshold.

Step 550, TRANSFORM RESPONSE SIGNAL, may include transforming thedetected currents IA, IB, IC into a rotor reference frame by applyingEq. 1 above so that the response signal 18 defines a characteristiccurrent derived from currents flowing in each phase of the motor 12.

Step 560, CHARACTERISTIC CURRENT≠EXPECTED VALUE +/−DIFFERENCE THRESHOLD,may include indicating a motor fault if the characteristic currentdiffers from the expected value 24 by more than a difference threshold,e.g. the threshold 404.

Step 570, DETERMINE PRESENT INDUCTANCE, may include determining apresent inductance value LA, LB, LC of a phase PA, PB, PC of the motor12 based on the response signal 18.

Step 580, PRESENT INDUCTANCE≠BASELINE INDUCTANCE VALUE +/−INDUCTANCECHANGE THRESHOLD, may include indicating a motor fault by jumping tostep 590 if the present inductance differs from a baseline inductancevalue by more than an inductance change threshold.

In summary, if the outcome of all of the tests is NO, then no motorfault is detected, so the method 500 ends (END) and the motor 12 isbelieved to be functional. However, if the outcome of any of the testsis YES (i.e. the test is not passed), then a motor fault is detected andthe method proceeds to step 590

Step 590, INDICATE MOTOR FAULT, may include activating a warning lightviewable by an operator of the vehicle, and/or initiating a reducedtorque mode for operating the motor 12 to avoid a complete failure ofthe motor 12 if possible.

Accordingly, a system 10, a controller 22 for the system 10, and amethod 500 of detecting faults in a motor is provided. Full voltage testsignals are applied to the motor instead of reduced voltage test signalsso dielectric breakdown and other faults can be more readily detected.The drive signal 16 is configured so that the motor 12 does not moveeven though the drive signal 16 is applying full operation voltages.

While this invention has been described in terms of the preferredembodiments thereof, it is not intended to be so limited, but ratheronly to the extent set forth in the claims that follow.

We claim:
 1. A system for detecting faults in a motor, said systemcomprising: a drive circuit configured to apply a drive signal to amotor; a detection circuit configured to detect a response signalgenerated when the drive signal is applied to the motor; and acontroller configured to determine a motor fault based on a comparisonof the response signal to an expected signal for the drive signalapplied to the motor, wherein the drive signal is selected to generate arotating magnetic field in the motor with a rotation-frequency greaterthan a maximum mechanical-response-frequency of the motor.
 2. The systemin accordance with claim 1, wherein the drive signal includes a ramp-upportion after the drive signal starts, a ramp-down portion before thedrive signal ends, and a test portion between the ramp-up portion andthe ramp-down portion.
 3. The system in accordance with claim 2, whereinthe ramp-up portion and the ramp-down portion are configured to reducecurrent transients.
 4. The system in accordance with claim 1, whereinthe detection circuit is configured to measure current flowing in one ormore phases of the motor, and the response signal is based on currentflowing in one or more phases of the motor during a test portion of thedrive signal.
 5. The system in accordance with claim 4, wherein theresponse signal is characterized as an average magnitude of currentflowing in a phase of the motor.
 6. The system in accordance with claim5, wherein the controller is configured to indicate a motor fault if theaverage magnitude is less than an open-circuit threshold.
 7. The systemin accordance with claim 5, wherein the controller is configured toindicate a motor fault if the average magnitude is greater than ashort-circuit threshold.
 8. The system in accordance with claim 4,wherein the response signal is characterized as a characteristic currentderived from currents flowing in each phase of the motor.
 9. The systemin accordance with claim 8, wherein the controller is configured toindicate a motor fault if the characteristic current differs from theexpected value by more than a difference threshold.
 10. The system inaccordance with claim 9, wherein the characteristic current is based ona transformation of the currents into a rotor reference frame.
 11. Thesystem in accordance with claim 1, wherein the controller is configuredto determine a present inductance value of a phase of the motor based onthe response signal during a test portion of the drive signal.
 12. Thesystem in accordance with claim 11, wherein the present inductance valueis determined based on a minimum value and a maximum value of currentflowing in the phase of the motor during the test portion of the drivesignal.
 13. The system in accordance with claim 11, wherein thecontroller is configured to compare the present inductance value to abaseline inductance value, and indicate a motor fault if the presentinductance differs from the baseline inductance value by more than aninductance change threshold.
 14. A method of detecting faults in amotor, said method comprising: applying a drive signal to a motor;detecting a response signal that arises when the drive signal is appliedto the motor; and determining a motor fault based on a comparison of theresponse signal to an expected signal for the drive signal applied tothe motor, wherein the drive signal is selected to generate a rotatingmagnetic field in the motor with a rotation-frequency greater than amaximum mechanical-response-frequency of the motor.
 15. The method inaccordance with claim 14, wherein the response signal defines an averagemagnitude of current flowing in a phase of the motor, and the methodincludes indicating a motor fault if the average magnitude is less thanthe open-circuit threshold.
 16. The method in accordance with claim 14,wherein the response signal defines an average magnitude of currentflowing in a phase of the motor, and the method includes indicating amotor fault if the average magnitude is greater than the short-circuitthreshold.
 17. The method in accordance with claim 14, wherein theresponse signal defines a characteristic current derived from currentsflowing in each phase of the motor, and the method includes indicating amotor fault if the characteristic current differs from the expectedvalue by more than a difference threshold.
 18. The method in accordancewith claim 17, wherein the characteristic current is based on atransformation of the currents into a rotor reference frame.
 19. Themethod in accordance with claim 14, wherein the method includesdetermining a present inductance value of a phase of the motor based onthe response signal; and indicating a motor fault if the presentinductance differs from a baseline inductance value by more than aninductance change threshold.